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User blog:Samuli.seppanen/Random thoughts on the most impressive cheiroballistra reconstructions
Today I was writing an email to a fellow ballista reconstructor when I realized I should probably make the email a blog post. So here are two of the most impressive ballista reconstructions/reconstructors right now as far a I know. = Helmut Schmidt Universität / Burkhard Meissner = Their reconstructions are very interesting as they don't use sinew at all. Nor nylon. They've tested various oiled animal and plant fibres and achieved quite impressive results: bolt velocity up to 86 m/s, range 480+ meters. Their most interesting hypothesis is this: they believe sinew was not typically used as spring cord material, at least not in inswingers. This is because it simply is not elastic enough for springs where * The spring is relatively thick compared to its length * The spring needs to be rotated a lot (=inswingers) I tend to agree with their reasoning for the most part. Plus I know how much work processing sinew is even for a small weapon like the cheiroballistra. Just think about how much work would it be to produce sinew springs for a one talent engine... it is simply not cost-effective enough even with slave labour. For cheiroballistra - assuming that P.H.'s torsion spring measurements are not modified - I think nylon/sinew is still usable. This is because the spring is very slim compared to any of the archaeological specimens (see Calibration formulae and archaeological finds). Further proof is that the amount of holes in field frame rings and washers in archaeological specimens is not enough to adjust the spring tension accurately enough (see below for more on this). Their results will be published in a book/article "soon". = Firefly by Nick Watts = Nick's nylon reconstruction, The FireFly, is a classic. His results have been a watershed in my opinion, with bolt velocities of 90 m/s+ and ranges beyond 800 meters. That said, I think the relative lack of elasticity in nylon has lead him to solutions that are questionable from a scientific reconsturction perspective. For example, the last time I checked Nick was using ~30 degree arm rotation even though his Firefly is an inswinger. Due to the shape of the force-draw curve of an inswinger it is possible to get fairly good results that way, if the spring has lots of linear pretension. However, a lot of the potential of the spring is still wasted - one could just as well make a ballista half the size and use all of the available rotation and draw length and get similar performance. So I think Nick has hit the limits of elasticity of nylon and thus went the above ~30 degree route. I had the same problem with reconstruction when I increased washer rotation - the spring itself started stacking and prevented increasing the draw length. The reason for spring stacking is also geometric - with enough rotation the spring cords start going from a | shape into a < shape. What this does is require the spring cords to stretch more and more for each unit of rotation. At some point the spring just stacks and forcing the arms to rotate more would break them (I've been there). So it is of utmost importance to have lots of linear pretensioning and minimal spring rotation to keep the force-draw curve as flat as possible. Use of very elastic spring cord material would also help a great deal. Nick was also forced to use a lot of holes in his field frame rings and washers to allow enough granularity in washer rotation for balancing the two springs. I've had the same problem myself, even with the cheiroballistra's very slender springs. Later Nick created an innovative solution for the problem (pairs of wedges on top of washer bars), but in many archaeological specimens it is clear that that solution would not work due to rounded top of the bars. Still, that kind of solution could have been used in the antiquity to increase linear pretension considerably in the field - there just is not any concrete proof of that. The top surfaces of the bars of the Lyon washers seem be flat based on the drawings by Dr. Baatz. If the surface are indeed flat and not rounded, then something round had to be on top, or the spring cords would get cut. That something could be a pair of wedges, the topmost of which would be rounded. Another explanation for Lyon washers is that the their flat bars could just be incomplete (i.e. not rounded yet). This is not unheard of, as the Gornea field frames very similarly incomplete. Category:Blog posts Category:Theoretical Category:Practical Category:Sinew Category:Torsion spring arming Category:Torsion